In general, proximity sensors are sensors configured for detecting and/or measuring the presence of an object close to the sensors without any physical contact. Such sensors are known to those skilled in the art and may be used for detecting the presence of an object or a gesture performed above a tablet, a cellular mobile telephone, or other similar electronic apparatus, with which an action is associated.
Radiation of optical light, e.g., infrared or laser light, is generally emitted towards an object close to the sensors to measure the Time-of-Flight (TOF) of this radiation. In other words, this is the time that elapses between light emission and reception by the sensor after reflection on the object.
However, the optical light detected by the sensors may be too low to generate an analog voltage representing the flow of optical light. In fact, the optical signal may include only a few photons per excitation/emission cycle. The desired time resolution is therefore often difficult to achieve with conventional electronic transient recorders.
One approach for addressing this problem is Time-Correlated Single Photon Counting (TCSPC). With a periodic excitation emanating, for example, from a laser, it is possible to extend a collection of data from the optical signal over several excitation and emission cycles. This approach is based on a precisely timed repetitive recording of each photon of optical light radiation, e.g., laser radiation, taking the optical period of the radiation as a time reference.
To this end, several types of single photon sensitive detectors may be used, such as Single Photon Avalanche Diodes (SPADs). In this regard, the histogram functionality is particularly useful for proximity sensors, e.g., of the SPAD type, for preparing precise timing information on the arrival of each individual photon resulting from optical light radiation.
Histogram use is a helpful feature for proximity sensors because histograms may be configured for complementing a closed circuit system, such as with proximity sensors, to operate as a TOF reading circuit coupled with a modulated light source and to provide additional information. In particular, this may allow detection of the presence of multiple objects.
However, the time resolution of histograms of SPAD proximity sensors is generally limited by the main internal clock having the highest frequency within the sensors. Histograms having a finer resolution may be prepared using specific high-frequency circuits, which are generally complex and costly in terms of silicon surface area.